1. Field of the Invention
The present invention relates a power system for converting DC output from a DC power source (such as a solar cell) into AC output by utilizing an inverter for power supply to an AC system. In particular, the present invention relates to a DC-AC conversion technique with the use of a plurality of inverters, for attaining efficient conversion of DC output into AC output.
2. Description of the Related Art
Referring to FIG. 6, the block diagram illustrates a conventional power system. As shown in the figure, the power system comprises a direct current power source DC (a solar cell), a first through a third inverters PS1–PS3 connected in parallel to the power source DC, a power detector DS for detecting the output power from the power source DC, an output controller SC for controlling the inverters in accordance with the output power, and switches SW1–SW3 for connecting or disconnecting the inverters with respect to the power source DC.
As shown in FIG. 6, the first inverter PS1 comprises a first inverter control circuit CO1 and a first inverter circuit PC1. The second inverter PS2 comprises a second inverter control circuit CO2 and a second inverter circuit PC2. The third inverter PS3 comprises a third inverter control circuit CO3 and a third inverter circuit PC3.
Upon receiving a start signal from the output controller SA, the first inverter control circuit CO1 starts to operate and close the first switch SW1, so that the power outputted from the power source DC is supplied to the first inverter circuit PC1. At the same time, the control circuit CO1 actuates the inverter circuit PC1, which converts the DC voltage into AC voltage to be supplied to the system power source AC. The second inverter control circuit CO2 and the third inverter control circuit CO3 operate in the same manner as described above.
The power detector DS detects the output power from the power source DC and inputs the result into the output controller SA. The output controller SA comprises an output power calculating unit CA and an inverter selector CH. The output power calculating unit CA periodically measures the output power at regular time intervals, and determines, based on the measurements, how many inverters to be actuated. Then, in accordance with the determined number of the inverters, the inverter selector CH sends a start signal to the relevant inverters.
FIG. 7 is a timing chart for describing the operation of the conventional system. In this system, when the output power from the DC source exceeds a predetermined reference value (not shown) at time t=t3, the output power calculating unit CA outputs a start signal for actuating one or more of the currently idle inverters. However, as shown in FIG. 7, the additional inverter(s) needs operation delay time T1 (several tens of seconds, for example) after the start signal is inputted, until its output supply becomes stable. Due to this operational delay, the output power indicated by hatching in FIG. 7 cannot be properly converted, whereby the system power source AC fails to receive an appropriate power supply.
As one way to address the above problem,
JP-A-H11(1999)-341816 teaches the following technique. While the current source DC (see FIG. 6) supplies power, this output power is measured at predetermined time intervals (each interval may be several tens of milliseconds, for example). Then, the differential quotient of the output power is calculated for the past few minutes, and it is determined whether or not the quotient tends to increase.
If the differential quotient is on an increasing trend, the output power at the next periodic point t=t3 is estimated based on the above quotient, and then it is determined whether or not the estimated power output is beyond the capacity of the currently operating inverter(s). When the estimated power output is beyond the current capacity, a start signal for actuating another inverter is outputted at the time t=t2. As a result, at t=t3, the operation of the additionally actuated inverter will be stable. In this manner, since the output power does not exceed the capacity of the inverters, the output power from the power source DC can be efficiently converted.
According to the above-described method, the estimation of the output power at the next periodic point can be performed accurately when the increase of the output power is moderate, as in the case shown in FIG. 8. As shown in FIG. 9, however, the output power may decrease sharply for the measurement time, and then may increase. In such a case, since the output power at the time t=t3 is estimated based on the decreasing differential quotient, the number of inverters to be actuated at t3 will be decreased, even though the actual output power will increase after the time t=t2. As a result, the output power at t3 exceeds the capacity of the inverters, whereby it cannot be properly supplied to the system power source AC.